1. Field of the Invention
This invention relates to a static power converter, and uses for the static power converter.
Static power converters are devices for converting electrical power, and include one or more sets of diodes, or sets of thyristors, and, as appropriate, a power-converter transformer, reactors, switching units, and further auxiliary devices. They are designed, inter alia, as rectifiers, inverters, and converters, and are used for operating electrical machines. During operation, static power converters are subject to current harmonics, which lead to undesirable feedback perturbations affecting the current-supply system. If no protective measures are adopted, dangerous overvoltages can occur in the process of switching off inductive loads, and both these loads and the semiconductor components of the power converters are particularly endangered by such overvoltages.
The state of the static power converter art is described in the Taschenbuch Elektrotechnik [Electrical Technology Pocket-Book], Volume 5, VEB Verlag Technik, Berlin, 1980, page 463 to 469. This literature reference describes a motor which is supplied from a static power converter. In order, over the run-up range, to guarantee the commutation of the inverter which is connected on the motor side, the current in the direct-current intermediate circuit is automatically adjusted to zero, under the control of a timing-pulse generator. For the purpose of support, a smoothing reactor can be short-circuited via a free-wheeling thyristor. Reverse operation is possible by reversing the polarity. Depending on the circuit arrangement, the static power converter control element can exhibit voltage-controlling behavior, or current-controlling behavior, and can be designed as a bridge circuit possessing thyristors in its arms, with facilities for turning these thyristors off. In order, during the process of switching off inductive loads, to avoid impermissible overvoltages at the semiconductor components of the power converter control element, it is necessary to adopt overvoltage protection measures which can be brought into effect quickly. A further problem is the imposition of the current harmonics, which occur during operation of a power converter, onto the supply system.
With regard to the relevant state of the art, attention is drawn, in addition, to the periodical published by the Swiss Brown Boveri Company [Brown Boveri Mitteilungen], 1982, page 157 to 162, and in particular to page 159, which discloses power converter circuits of the twelve-pulse type, each possessing a separate converter for each part-winding of a machine, that is to say of a converter-supplied synchronous motor, as well as a power-converter circuit in association with a single-winding machine and a three-winding transformer, connected on the motor side, for use as a starting converter. In these circuits, two converters are used, in each case, with d.c. intermediate circuits and simple thyristors in the bridge arms. On the line side, they require static converter transformers with two secondary windings, which are electrically shifted through 30.degree.. On the machine side, the stator winding must either be designed to have two separate groups (double-wound machines), or, if the motor is of single-winding design, a three-winding transformer must also be employed on the machine side.